This invention relates to devices capable of keeping a rotating shaft on speed in the event of a transient disconnection from the electrical supply of the motor driving the said shaft.
A particularly interesting application of this invention concerns the drive of a liquid fuel pump of a combustion turbine.
Generally, in an industrial application, the accessories of the combustion turbine are electrically supplied by two different sources to avoid a common failure mode.
For reliability reasons, these two sources are not supplying the accessories at the same time so that possible feeder fault is not communicated from one source to the other, for example a voltage drop caused by an overload or a short circuit.
The transfer from one source to the other is done with a transient disconnection from the supply. This transfer from one source to the other is done automatically with a fault detection on the source in use, or by operator action, for example operation or maintenance reasons.
Such transfer done without specific precautions, can be particularly disturbing for the operation of the combustion turbines. As a matter of fact, it is mandatory to keep the supply of the high pressure liquid fuel to alleviate the risk of trip by loss of flame or to disturb the power production. For certain countries, the grid codes call for an obligation for the combustion turbine to be capable of continuing operation in the event of a power loss of 1.5 second, for example this is the case in the Netherlands.
To comply with this obligation, the use of a pneumatic accumulator having a reservoir which volume is separated by an elastic membrane with high pressure liquid fuel on one side and a pressurized neutral gas on the other side is known. This arrangement performs well on the low pressure liquid fuel side. However, the accumulator interferes in an unacceptable manner with the control of the turbine when used on for high pressure liquid fuel as it results in increased time constant. The turbine is then unable to face rapid load variations for example further to the opening of a breaker or on an isolated grid, the adjustment of the fuel flow being unable to follow accurately the load variation.
Furthermore, the filling flow of the accumulator when starting, or during the restoration of the accumulation further to a source transfer, or further to a sudden load variation of the turbine is taken off the fuel pump capacity, this affects the capability of the pump to deliver the required flow to ensure the proper operation of the turbine. This results in an extension of the transient time further to a source transfer or other disturbances.
Similarly, it is known to use a mechanical accumulator of a type having piston and cylinder assembly that provide a first chamber in which is the high pressure liquid fuel of the turbine supply and a second chamber filled with an actuating fluid.
In operation, the piston is completely pushed by the fuel pressure. In normal operation, this device has no action as the piston rests on the cylinder bottom when the storage is completed, and does not interfere with the fuel control as does the pneumatic accumulators. Following a transient disconnection from the power supply, the actuating fluid present in the accumulator causes the piston displacement, which allows for restitution of a stored fuel quantity into the turbine supply thus backing up the pump flow reduction.
This arrangement has the major drawback of being not suitable for industrial applications, for which the fuel pressure may vary continuously to cope with the operating conditions. As a matter of fact, the mechanical accumulator fuel restitution is done at the pressure it has been stored which corresponds to only a specific operating point. Thus, during a transient disconnection, a load variation is observed as big as the difference between the storage conditions and the operating conditions prevailing at the event time.